The market demand for dustless and free-flowing chemical products has increased significantly and is likely to continue in this trend. The demand for dust-free, flowable materials is especially high in the compounding of rubber and polymeric compositions.
The use of finely divided materials in compounding of natural and synthetic rubber presents dusting problems which may have adverse consequences relative to productivity, contamination, environmental pollution, and safety. The finely divided nature of most rubber additives causes them to be blown into the air during production operations such as conveying, mixing or milling. This results in a production efficiency reduction due to the loss of some additives. Additionally, the dusting problem can result in the possible contamination of other materials. Further, airborne dust causes environmental and housekeeping problems, as well as safety hazards created by the explosive nature of finely divided dust. Bridging problems of powder materials also adversely affect their use in automatic feeding systems. Thus, it is desirable to have rubber and polymer additives in a dustless, free-flowing form with good dispersion properties so that processors can meet environmental regulations, application needs, and implement automatic feeding systems for improved productivity.
Efforts have been made in the art to eliminate the problems associated with finely divided materials by converting such materials into agglomerates. To form such agglomerates, generally a binder must be added to the finely divided materials. Frequently, this binder has been a material which was objectionable when mixed into a rubber compound with which the finely divided material was used. Some of the prior art agglomerates were cohesive and thus not free-flowing, and do not lend themselves to normal or automatic handling and weighting procedures. Some materials employed for preparing prior art agglomerates were used in such large amounts that they diluted the rubber ingredients or did not readily disperse in rubber during mixing. Such binders are generally classified as matrix-type binders wherein the binder material fills the space between the solid particles throughout the agglomerate resulting in the solid particles of the chemical powder additive imbedded in a more or less continuous matrix. The amount of matrix-type binder required for chemical products for the compounding of rubber and polymeric compositions is in the range of about 10% to about 20% or higher. Matrix-type binders are generally water insoluble since a water soluble matrix-type binder would be dissolved in an aqueous solution. An example of such a matrix-type binder is found in U.S. Pat. No. 3,362,913. It has been found that such matrix-type binders remain in the chemical product and are found in the final product at a high level which is undesirable in some applications.
Minimum amounts of the binder in agglomerates are desirable for shipping and dispersion. The amount of binder utilized can be minimized by the use of film-type binders wherein the binder encapsulates the particle and does not fill the space between the particles resulting in an average decreased distance between particles.
However, for pellets, the amount of binder is limited by a required physical property called the pellet crust strength. The pellet strength is measured by applying weight on the pellet until it is crushed and then dividing the total weight applied by the cross section area of the pellet where the weight is applied to. A desirable range of crush strength suitable for both shipping in bags and dispersion is about 10 to about 35 gm/mm.sup.2.
In addition, the chemical powder additive must completely disintegrate into its component particles when it is mixed with the rubber or other plastic substances under pressure and dispersed homogeneously therein. Also, the added binder must not impair the stability of the chemical powder additive and the characteristics of the rubber or plastics.
In many of the prior art agglomerates, the pellet crush strength was either too low or too high. Agglomerates with too low of a crush strength are undesirable because of their inability to withstand normal shipping and handling. Agglomerates having too high of a crush strength generally had poor dispersing properties and did not mix well. Typical processes for agglomerating powder materials are disclosed in U.S. Pat. Nos. 4,362,559; 4,194,901; and 4,298,858; Canadian Patent No. 890,833; Japanese Patent Nos. 78 90,116; 78 66,923; 8,205,731 and 82,192,439; and Czech Patent No. 185,383.
It is the objective of the present invention to solve the above problems associated with powder materials and prior art agglomerates by providing a pelletization process for making small, dustless, and free-flowing products which have good dispersion properties. It is a further objective of the present invention to produce pellets of chemical additives which have a very low level of binders and other processing aids. A major advantage of the present invention is the production of pellets with a pellet crush strength range suitable for both shipping and effective dispersion, which is controllable to customer specification.